1. Field of the Invention
The present invention relates to a strain sensor using a fiber Bragg grating to sense tension or compression.
2. Description of the Related Art
A fiber Bragg grating (FBG) is a type of Bragg diffraction element comprising an optical fiber with a core having a periodically varying refractive index, so that regions having a high refractive index NH alternate with regions having a low refractive index NL along the fiber axis. Light propagating through the fiber is reflected back if its wavelength is equal, or approximately equal, to the Bragg wavelength xcexB which is expressed as follows in terms of the effective refractive index ne of the core and the grating pitch d (the distance between successive regions with the same refractive index).
xcexB=2nedxe2x80x83xe2x80x83(1) 
An FBG may have a uniform grating pitch, or it may be chirped. In a chirped FBG, the grating pitch d varies, either continuously or in stages, along the length of the fiber. The transmission and reflection characteristics of an FBG depend on the presence or absence of chirp and on such grating parameters as the grating count (the number of alternating regions) and the degree of modulation of the refractive index (the difference xcex94n between NH and NL). For example, the maximum reflectivity of an FBG increases with xcex94n; for a given grating pitch d, the maximum reflectivity increases with the grating count; for a given grating count, the width of the reflection band increases with increasing chirp.
If an FBG is subjected to variations in temperature or strain, the effective refractive index ne and the grating pitch d in equation (1) change, altering the Bragg wavelength. An FBG can therefore be used as a strain sensor or a temperature sensor, by detecting the Bragg wavelength.
Strain sensors are useful in what is termed smart structure technology, in which sensors are built into buildings, bridges and other structures to sense changes in strain over time at various points. Since an FBG is sensitive to both strain and temperature, for use as a strain sensor, it must be temperature-compensated. One general method of temperature compensation that is being studied employs chirp to create a temperature-independent reflection band.
Japanese Unexamined Patent Publication No. 2000-97786 discloses several strain sensors employing this general type of temperature compensation. One of these conventional strain sensors will be described in detail here for comparison with an embodiment of the present invention to be described later. The conventional strain sensor described here, shown in a top plan view in FIG. 1 and in a side view in FIG. 2, has an optical fiber 10 with an FBG 12 attached by an adhesive, for example, to a tension member 44. The tension member 44 has the general form of a rectangular plate with a tapered section 44a at or near the center where the FBG 12 is located. The axis xcex1 of the optical fiber 10 extends longitudinally through the tapered section 44a. If longitudinal tension stress is applied to the tension member 44, then the tapered section 44a elongates by an amount that increases with decreasing width of the taper. The FBG 12 elongates in a similar manner. As a result, the grating pitch of the FBG 12 increases toward the narrow end of the tapered section 44a, changing the FBG 12 from a uniform grating to a chirped grating.
FIG. 3 shows how the reflection band of the FBG 12 changes in response to strain. The horizontal axis indicates wavelength; the vertical axis indicates the relative optical power of the reflected light. Reflection spectrum 62, which has a reflection band 64, is observed before a certain tension force is applied; reflection spectrum 66, which has a wider reflection band 68, is observed after the tension force is applied. The amount of strain caused by the tension can be determined from the width of the band from xcexmin to xcexmax in which the reflected optical power is equal to or greater than a certain quantity. The change in this bandwidth is independent of temperature, so measurement of this bandwidth, or of the change therein, provides a way to measure strain without interference from temperature effects.
The strain xcex5max in the narrowest part of the tapered section 44a (the maximum strain), the strain xcex5min in the widest part of the tapered section 44a (the minimum strain), the tension force F, the minimum cross-sectional area AS of the tapered section 44a, the maximum cross-sectional area AL of the tapered section 44a, and Young""s modulus E are related by the following equations (2) and (3).
xcex5max=F/(Exc2x7AS)xe2x80x83xe2x80x83(2) 
xcex5min=F/(Exc2x7AL)xe2x80x83xe2x80x83(3) 
FIG. 4 plots the changes in xcex5max and xcex5min, shown on the vertical axis, as functions of the applied tension force F, shown on the horizontal axis. As the force F increases from F1 to F2, the maximum strain xcex5max and minimum strain xcex5min both increase proportionally. As implied by equations (2) and (3), however, the slope of the xcex5max characteristic 52 is greater than the slope of the xcex5min characteristic 54.
The grating pitch dmin in the widest part of the tapered section 44a (the minimum grating pitch) and the grating pitch dmax in the narrowest part of the tapered section 44a (the maximum grating pitch) are related to the grating pitch d0 when there is no strain by the following equations (4) and (5).
dmax=(1+xcex5max)d0xe2x80x83xe2x80x83(4) 
dmin=(1+xcex5min)d0xe2x80x83xe2x80x83(5) 
FIG. 5 plots the grating pitch d, shown on the vertical axis, as a function of longitudinal coordinates on the tension plate 44, shown on the horizontal axis. The solid curve 56 indicates the grating pitch d when a comparatively large tension force (e.g., F2) is applied; the dash-dot curve 58 indicates the grating pitch d when a smaller tension force (e.g., F1) is applied. The coordinates x1 and x2 in FIG. 5 correspond to the positions of the two ends of the FBG 12 in the optical fiber 10. The tapered section 44a of the tension plate 44 is widest at position x1, where the minimum grating pitch dmin occurs, and narrowest at position x2, where the maximum grating pitch dmax occurs.
When a tension force F is applied, the resulting elongation of the FBG 12 varies continuously from one end x1 and to another end x2 of the FBG 12, increasing from the widest end to the narrowest end of the tapered section 44a. The grating pitch d therefore varies continuously, as shown by curve 56 in FIG. 5. As the tension force F increases from F1 to F2 in FIG. 4, the maximum strain xcex5max and minimum strain xcex5min in the tapered section 44a both increase proportionally, and the maximum grating pitch dmax and minimum grating pitch dmin increase according to equations (4) and (5), causing the upward shift from curve 58 to curve 56 in FIG. 5. The difference between the maximum grating pitch dmax and the minimum grating pitch dmin determines the total chirp, and also determines the rate of change in the grating pitch d in the longitudinal direction.
The change xcex94xcex in the reflection bandwidth can be understood in terms of the Bragg wavelength xcexmax at the end of the FBG 12 with maximum strain and the Bragg wavelength xcexmin in at the end of the FBG 12 with minimum strain. From the formula for the Bragg wavelength, these Bragg wavelengths are given by the following equations (6) and (7).
xcexmax=2nexc2x7(1+xcex5max)d0xe2x80x83xe2x80x83(6) 
xcexmin=2nexc2x7(1+xcex5min)d0xe2x80x83xe2x80x83(7) 
Since characteristic 52 in FIG. 4 has a greater slope than characteristic 54, when tension force is applied, the Bragg wavelength xcexmax at the end of the FBG 12 with maximum strain, corresponding to the maximum grating pitch dmax, increases more than the Bragg wavelength xcexmin at the end of the FBG 12 with minimum strain, corresponding to the minimum grating pitch dmin. As the tension force F increases, the difference between xcexmax and xcexmin therefore widens, causing an increasing change xcex94xcex in the reflection bandwidth.
From these results, the reflection bandwidth, or more precisely, the change xcex94xcex in the reflection bandwidth relative to the unstressed state, can be plotted against tension force F as in FIG. 6; as the tension force F increases from F1 to F2, xcex94xcex increases proportionally; strain is measured by measuring xcex94xcex.
The size of the change xcex94xcex in the reflection bandwidth in relation to the change in tension force F determines the sensitivity of the measurement. That is, the sensitivity can be expressed as the slope of the line in FIG. 6, or as xcex94xcex/xcex94F.
The sensitivity is affected by the ratio of the maximum cross-sectional area AL to the minimum cross-sectional area AS in the tapered section. That is, the sensitivity of the sensor is determined by the geometry of the taper. The sensitivity of this type of sensor is therefore limited by practical constraints on the taper geometry. From the standpoint of engineering design as well as sensitivity, it is undesirable for the performance of the sensor to be restricted by geometrical constraints.
An object of the present invention is to provide a strain sensor with improved sensitivity.
Another object of the invention is to provide a strain sensor with a sensitivity that is not limited by geometrical constraints.
The invention provides a fiber Bragg grating strain sensor including a strain sensor member having a strain sensing section for receiving stress in a longitudinal direction. An FBG is fastened to the strain sensor member within the strain sensing section. At one end, the FBG is oriented in the longitudinal direction of the strain sensing section. At the other end, the FBG is oriented at a right angle to the longitudinal direction. Between these two ends, the FBG describes one quarter of a circular arc.
When longitudinal stress is applied to the strain sensing section, the FBG is elongated at one end and compressed at the other end. The combination of compression and elongation increases the amount of chirp created within the FBG, thereby enhancing the sensitivity with which strain can be measured. The sensitivity depends not only on the shape of the strain sensing section but also on the dynamic properties (Poisson""s ratio) of the material from which the strain sensing section is made. The sensitivity of the strain measurement can thus be improved through selection of a material with desired dynamics, which are not subject to geometrical constraints.